1. Field of the Invention
This invention relates generally to wire bonding lead frames to semiconductor dice and, more specifically, to wire bonding a lead frame to a semiconductor die using a laser beam to provide the energy necessary to bond or fuse a wire to a lead of a lead frame and to a contact pad of a semiconductor die. The method and apparatus of lead-to-chip bonding herein described are applicable to either a conventional lead frame and chip arrangement or a lead-over-chip (LOC) arrangement, in any instance, where the lead of a lead frame is directly or indirectly bonded to the contact pad of a semiconductor chip.
2. State of the Art
Various types of semiconductor chips are connected to lead frames and subsequently encapsulated in plastic for use in a wide variety of applications. A conventional lead frame is typically formed from a single continuous sheet of metal, typically by metal stamping operations. The conventional lead frame includes an outer supporting frame, a central semiconductor chip supporting pad and a plurality of leads, each lead having, in turn, a terminal bonding portion near the central chip supporting pad. Ultimately, the outer supporting frame of the lead frame is removed after the wire bonds between the contact pads of the semiconductor chip and the leads are made and the semiconductor chip and lead frame have been encapsulated.
In a LOC lead frame the lead frame has no central chip supporting pad with the semiconductor chip being held in position with respect to the lead frame and leads by means of adhesive strips secured to the leads of the lead frame and the semiconductor chip.
A typical apparatus and method for forming the wire bonds between the contact pads on a semiconductor chip and the leads of lead frames are illustrated in U.S. Pat. No. 4,600,138. As disclosed, a bond head is shown moving from a first bonding location to a second bonding location. The end of the wire is bonded to the first bonding location by the bond head. The bond head moves vertically away from the first bonding location to draw a length of wire necessary to make the wire bond. The bond head is then moved to the second bonding location with subsequent bonding of the wire to the second bonding location. The bond head is then used to pull and subsequently break away the remaining wire from the second bonding location. The bond head is then ready to be moved to another first bonding location for effecting another wire bond.
Typically, the bond head is heated to assist the formation of the wire bond. The heat and subsequent pressure applied by the bond head fuses the end of the wire to the contact pad. Ultrasonic vibration in conjunction with a heated bond head may also be used to affect a wire bond. Typically, there is a single bond head for making all of the wire bonds of the semiconductor chip. As should be recognized by those skilled in the art, such an operation is inherently mechanical in nature and thus limited to the speed of movement of the mechanical device.
One method of speeding up a conventional wire bonding process is to provide the heat necessary to effect a wire bond by utilizing heat generated from a laser beam to heat the bond head. Such apparatuses are disclosed in U.S. Pat. No. 4,534,811 to Ainslie et al., and U.S. Pat. No. 4,845,354 to Gupta et al. However, as the number of connections per semiconductor chip increase and the size of the leads decrease, such a bonding tool becomes impractical.
It has also been recognized in the art to use laser beams to form a lead-to-chip bond. For example, a method for reflowing solder to bond an electrical lead to a solder pad using a laser, in which the solder pad, rather than the terminal, is irradiated by the laser beam, is disclosed in U.S. Pat. No. 4,926,022 to Freedman. In addition, in U.S. Pat. No. 5,274,210 to Freedman et al., electrical connections may be made by coating conductive elements with a non-flux. nonmetallic coating, material making it possible to use a laser for bonding. The laser is either moved in a continuous sweep around all of the connections or pulsed.
It has also been recognized in the art to use a laser beam to bond the bumps of an integrated circuit to a tape automated bonding (TAB) tape lead. TAB, in general, has been one attempt in the art to increase the speed and efficiency of the chip-to-lead bonding process. For example, in U.S. Pat. No. 4,978,835 to Luijtjes et al. and U.S. Pat. Nos. 5,049,718 and 5,083,007 to Spletter et al., a laser beam is directed onto the ends of the leads of a TAB tape.
None of the previously mentioned prior art references, however, have successfully utilized laser light to reduce the mechanical limitations of the bonding process. More specifically, prior art devices either move the device relative to the laser for every bond or move a single laser beam to every bonding site. Thus, it would be advantageous to provide an apparatus and method for forming wire bonds using a laser in which the laser need not move for each bond and where more than one bond can be made substantially simultaneously.
Accordingly, the present invention provides a bonding apparatus and method of using the same for bonding any lead frame, either a conventional lead frame or a lead-over-chip (LOC) lead frame, to a semiconductor chip. Preferably, the semiconductor chip will include at least one contact pad on its active surface for providing an output, or input, as the case may be, of the chip. Likewise, the lead frame will include at least one lead to be connected to the contact pad of the chip. The chip/lead frame arrangement may be one where wire bonding is necessary to make the electrical connection between the contact pad and the lead, or a LOC arrangement where the lead of the lead frame extends over the active surface of the chip and is bonded to the contact pad with a short wire or a bump of solder. In either case, the bond required to make the connection uses an energy beam from a beam-emitting energy source to provide the energy necessary to make the connection.
In a chip/lead frame arrangement where a wire bond is used to make the electrical connection, the wire is aligned with the contact pad and the lead is subsequently bonded or fused to each using a directed energy beam. A beam of energy is focused on the site of the bond with a lens or plurality of lenses. Moreover, the wire may be directly bonded or fused to the contact pad and lead by melting the wire with the energy beam, or an energy fusible, electrically conductive bonding material may be provided proximate the bonding site.
In a preferred embodiment, the wire bonds at the contact pad and at the lead are substantially simultaneously bonded or fused. This may be accomplished by using more than one beam emitting energy source to direct more than one beam of energy, each to a different bonding site, or providing a single beam emitting energy source and splitting the beam into more than one smaller beam and directing the smaller beams to different bonding sites.
The beam emitting energy source used in conjunction with the present invention may be of various types known in the art. For example, the energy source may emit a laser beam, such as that produced by a pulsed solid state laser, a carbon dioxide laser, a Nd:YAG laser, or a Nd:YLF laser, a focused beam of light, a beam of radiant energy such as an electron beam, or a heat source, etc. In any case, the energy beam is preferably directed to the bonding site by prisms, mirrors, fiber optics, lenses and/or other reflective and/or deflective surfaces or combinations thereof. More specifically, in one preferred embodiment, each beam of energy is directed by prisms or mirrors from the energy source toward each bonding site. At least one lens is provided between each bonding site and the prisms or mirrors to further focus the beam of energy onto the bonding site. Each lens may be individually supported by a frame-like support structure or contained within an optical flat in which a plurality of lenses is formed. For a frame-like structure, the lenses may be moved and/or oriented to accommodate different chip/lead frame configurations. If the lenses are contained in an optical flat, a different optical flat may be used to accommodate various chip/lead frame configurations. In addition, the reflective and/or deflective surface may be articulatable to direct the energy beam to various bonding sites. With such an articulatable configuration, various configurations of lead frames and chips can be accommodated with the present invention.
In use, the energy beams are directed to a first set of bonding sites until the heat generated from the energy beams creates the bonds or fuses the bonds (i.e., wire bond or LOC bond) associated with the first set. The apparatus then translates the energy beams relative to the chip to a second set of bonding sites to make a second set of bonds. This process is repeated until all of the bonds associated with the chip/lead frame arrangement are formed.
An indexing system may also be associated with the apparatus to index chips. lead frames and other components into and out of the bonding location. The indexing system may comprise conveyors, articulating arms, magazines for housing the semiconductor device components, and other equipment known in the art. In addition, the entire system, from controlling the operation of the energy source to controlling which set of bonding sites is bonded to indexing the semiconductor device components, is controlled by at least one or more microprocessors.
As previously mentioned, a semiconductor chip bonded to its associated lead frame, in accordance with the present invention, may have a conventional configuration where the contact pads are positioned proximate the periphery of the chip or a LOC configuration where the contact pads are positioned closer to a center line of the chip. In either case, in a preferred embodiment, a thin flexible dielectric material (foil) containing fully or partially embedded wires may be placed, at least partially, over the surface of the chip containing the contact pads and the leads of the lead frame. When properly positioned, the wires extend from the contact pads to the leads of the lead frame. The foil may be adhesively attached to the chip and/or lead frame, held in place by a slight vacuum, or retained by a suitable clamping device in order to maintain proper alignment of the wires relative to the chip and lead frame. At the ends of each wire, an energy bondable, fusible, electrically conductive material (such as solder) may be provided for bonding the ends of the wire to the semiconductor chip and lead frame. Similarly, the energy bondable, fusible, electrically conductive material may be attached to the contact pads of the chip and/or the leads of the lead frame prior to positioning of the foil such that the energy fusible, electrically conductive material may be heated and subsequently bonded to the ends of each wire.
For a LOC configuration where the leads of the lead frame extend over the contact pads, an energy bondable, fusible, electrically conductive material (e.g., solder) may be provided between the lead and the contact pad. The solder may be bumped onto the contact pads by methods known in the art or attached to the ends of the leads to define a protuberance on the end of the lead so that when the lead frame is superimposed over the chip, the protuberance of solder is positioned above each contact pad. The solder may be bonded or fused to make the electrical connection between the leads and the contact pads by heating the leads themselves with an energy beam or providing leads that define openings through which the beam may be directed directly onto the solder. In yet another preferred embodiment, the ends of the leads themselves may be configured to contact the contact pads and may be bonded directly thereto by heating the lead.
A preferred embodiment of a semiconductor device, manufactured according to the present invention, would comprise a semiconductor chip having a plurality of contact pads, a lead frame having a plurality of leads, a foil layer or other suitable type material having a plurality of wires at least partially embedded therein, and a laser-bondable, electrically conductive material making the electrical connections between the wires and the contact pads and leads.
Although the bonding apparatus of the present invention has been described in relation to several preferred embodiments, it is believed that a major advantage of the apparatus, according to the present invention, is the efficient use of a beam emitting energy source, such as a laser, to quickly and efficiently bond a lead frame to a semiconductor chip by reducing the mechanical movements generally associated with prior art bonding apparatuses. This and other features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and as defined by the appended claims.